DMAs (Differential Mobility Analyzers) classify charged particles based on their capacity to migrate in an electric field. A DMA basically works by introducing a charged particle sample in a clean gas flow, which drags them through a region where there is an electrical field. The combination of the electrical and the flow fields classify the particles as a function of their electrical mobility. At the same time, this is a function of the particle size and charge. Knowing the particle charge distribution, it is possible to determine the particle size distribution.
DMAs have been traditionally used in measuring the particle size distributions in environmental applications such as atmospheric aerosols and combustion emissions. The traditional size range has been 15-1000 nm. Modern nanotechnology and other analytical applications require extending this range down to 1 nm with high resolution, high enough to classify very close particle sizes. Some nanomaterial properties drastically depend on the particle size used during the material synthesis, e.g., the particles must be highly monodisperse (having the same size). This can only be reached with high resolution instrumentation. So, the DMA characteristics are defined not only by the particle size range, but also by the resolution reached in that range (Rosser and Fernández de la Mora, 2005).
In the case of non diffusive particles, the ideal instrument resolution comes from the ratio between the sheath flow rate and the aerosol flow rate (Knutson and Whitby, 1975). However, the limiting factor to reach high resolution in the nanometric range is Brownian diffusion. A possible strategy to minimize the diffusive effect is to increase the Peclet number, therefore, increasing the Reynolds number for a selected particle size (de Juan et al., 1998). However, at the same time the flow must be kept under a laminar regime. In case of appearance of turbulence, the instrument resolution would be drastically degraded.
An additional condition is that the gas flow must be axilsymmetric. In the original Winklmayr design, the sheath flow rate is radially extracted. To keep the flow axilsymmetry it is necessary to have a constriction downstream of the aerosol outlet slit. This means a high pressure drop, which preempts high flow rates and high resolutions. An attempt to solve this problem was that of Rosser and Fernández de la Mora (2005) and the Martinez Lozano et al. (2004) previous prototype, where there were two outlet chambers to avoid the asymmetry caused by the conventional outlets in DMAs. When duplicating the chambers, it is necessary to include a similar constriction to the traditional ones, increasing the pressure drop in the sheath flow outlet. All these problems have forced the use two different DMAs to cover the 1-600 nm size range, because one instrument was not able to cover it with enough and adequate resolution.
In the proposed system, the present invention, the sheath flow outlet is produced with no constriction in the chamber. At the same time, the sheath flow outlet is completely axial and allows to have relatively low pressure drop and to reach high Reynolds numbers. This pressure drop is minimized by not using diffusers and with parallel walls, without divergent angles. All of this produces an instrument able to measure in the range 1 to 600 nm, with resolutions higher than 10 and keeping the aerosol inlet and outlet slit distances as in the Winklmayr DMA. Further, the sheath gas extraction system is very easy to machine and assemble.